This invention relates to imaging systems and more particularly to liquid development systems and liquid developers for use in color reproduction utilizing multiple development.
Color electrophotography with multiple development is capable of producing color reproductions by the following exemplary procedures. A suitable photoconductor such as substantially white zinc oxide photosensitive paper, Electrofax paper for example, is electrostatically uniformly charged in the dark and then exposed through a green filter to an imagewise projection of a color image to form an electrostatic latent image on the photoconductor. The electrostatic latent image is then developed with magenta colored toner to form a magenta colored image corresponding to said electrostatic latent image. The zinc oxide photosensitive paper is again electrostatically uniformly charged in the dark and then exposed through a red filter to an imagewise projection of a colored image in register with said magenta developed image to form a second electrostatic latent image, which second image is developed with cyan colored toner. Similarly, the zinc oxide photosensitive paper is again electrostatically uniformly charged in the dark and then exposed through a blue filter to an imagewise projection of a colored image in register with said magenta and cyan developed images to form a third electrostatic latent image, which is then developed with yellow toner to complete a reproduced color image.
The sequence of exposures through colored filters in this multiple development process may be performed in any suitable sequence other than the green, red and blue sequence recited above.
A significant drawback of this multiple development process is that after the formation of the image of the first color and during the second imaging sequence consisting of uniformly charging and imagewise exposing followed by development with toner of the second color, the zinc oxide photosensitive paper is apt to be electrostatically charged more strongly in the portion where said first colored image is formed in comparison with the other portion where such image does not exist. In addition, the portion of the zinc oxide paper where the first colored image is formed is apt to retain charge in nonimage areas when imagewise exposed to a light pattern which is capable of neutralizing the electrostatic charge in the latter portion. This retained potential, which usually ranges from several volts to several tens of volts, arises from the fact that the ion absorbed by the toner during charging is not neutralized during the imagewise exposure to light. Since the toner usually consists of electrically insulating material the neutralization of the ion for example, held by the toner layer, for material the corona ion generated by corona discharge is hindered. Electroconductive toner cannot be employed in electrophotography with multiple development since the portion of the photoconductor having such toner on its surface during the second and third imaging sequences cannot bear electrostatic charge.
Furthermore, when the electrostatic charge on the first toner layer is not completely neutralized, the toner of second color tends to be improperly deposited onto the first toner layer, giving rise to impure color formation. Similar difficulties also arise in the development with the toner of the third color, and the tendency for improper toner deposition increases as the reflective optical density of the toner image already present is increased. The result of these characteristics is that it is very difficult to obtain color reproduction of satisfactory quality.
These difficulties have been lessened to some degree by the use of the techniques and materials disclosed in U.S. Pat. 3,060,020 which is herein incorporated by reference. Essentially therein disclosed is a technique utilizing toner chiefly consisting of photoconductive zinc oxide powder in order to provide appropriate photoconductive property to the toner image. This technique may be more fully understood by reference to FIGS. 1, 2A and 2B of the accompanying drawing in which:
FIG. 1 is an enlarged cross section of the toner particles.
FIG. 2A is an enlarged cross section of a toner image on an imaging surface.
FIG. 2B is an enlarged cross section of a fused toner image on an imaging surface.
In FIG. 1, toner particle 10 consists of core 11 composed of photoconductive zinc oxide particle surrounded by a colored resin layer 13, which may be composed either of pigment particles 12 dispersed in resin as shown or of resin colored with an appropriate dye. The resin 13 is required to be liquified by heat, and the melting point thereof is usually required to lie between about 90.degree. and about 250.degree. C. In addition, the resin layer is required to be highly insulating and to have sufficient capability to generate favorable frictional electricity (i.e. a capability to generate sufficiently strong positive charge if the latent image is negative). Furthermore the resin 13, when melted, should be of sufficiently low viscosity, perferably between about 45 and about 10,000 centipoises, so as to be removed from the surface of the zinc oxide core.
FIGS. 2A and 2B show the method of using the dry powder toner described in said U.S. Pat. in a dry development system. As shown in FIG. 2A, the toner image 21 is formed by toner particles 10 held onto the imaging surface 22 bearing an electrostatic latent image. The toner layer thus formed simply by means of electrostatic forces of attraction does not possess photoconductivity due to the high electric resistance of resin layer 13 surrounding the zinc oxide core 11. When the toner is melted by heat as shown in FIG. 2B, the resin 13 together with pigment 12 is spread onto the imaging surface thereby exposing the surface of zinc oxide core particle. As a result, the fixed toner layer 21' shown in FIG. 2B acquires photoconductive property on account of the exposed zinc oxide particles 11.
This process, however, has several drawbacks among which are the fact that the heating up to 90.degree. - 250.degree. C. required for melting the toner image may cause irreversible dilatation of the imaging surface which may result in the formation of unsatisfactory prints due to imperfect registration during the second and third imaging sequences. This difficulty is especially pronounced when the support material consists of paper, as for example in Electrofax paper. In addition, the colors obtained by this process are not of high saturation but rather become whitish since the white zinc oxide powder is almost exposed to the surface after fixing of the toner image by heat. This difficulty results in impure color or lack of color density when three color images are superimposed one upon the other. The melting point and limited viscosity range of the resin seriously confine the selection of suitable materials to only certain types which also must be highly insulating and capable of being triboelectrically charged to a suitable polarity and potential. Furthermore, this dry developer toner cannot be used with a particle size smaller than a certain limit, and therefore is not capable of providing high resolution and satisfactory tone reproduction. Actually, the toner is frequently composed of aggregate of several to several tens of zinc oxide particles instead of being composed of a single particle as shown in the ideal case of FIG. 1.